Of course it's not user expandable, how they will make money. Very few people will buy 128GB version and SSD increase will have very premium cost, like usual. But you should be able to use external disks via thunderbolt if you want to.
There's really no excuse for that these days. So disappointing that Apple must bake in this kind of greed into the product. Overall I think it's shortsighted because it will drive away many budget conscious customers.
I guess you could expand it with an external SSD via the USB or thunderbolt ports? Not ideal, but a potential workaround.